Analyze FieldAccessDescriptors and drop fields that are never accessed

We can walk backwards through the graph, analyzing which fields are accessed. When we find paths where many fields are never accessed, we can insert a projection transform to drop those fields preemptively. This can save a lot of resources in the case where many fields in the input are never accessed.

To do this, the FieldAccessDescriptor information must be added to the portability protos. 

Imported from Jira [BEAM-4457](https://issues.apache.org/jira/browse/BEAM-4457). Original Jira may contain additional context.
Reported by: reuvenlax.
Subtask of issue #18898